STRUCTURALLY DISTINCT DISINTEGRINS CONTORTROSTATIN AND MULTISQUAMATINDIFFERENTIALLY REGULATE PLATELET TYROSINE PHOSPHORYLATION

Tyrosine phosphorylation of multiple platelet proteins is regulated by the integrin alpha(IIb)beta(3). In order to further examine integrin- regulated tyrosine phosphorylation, we have used small Arg-Gly-Asp-con taining snake venom proteins (termed disintegrins) that inhibit platel et aggregation to competitively block the agonist-induced binding of f ibrinogen to alpha(IIb)beta(3). One structurally unique disintegrin, c ontortrostatin (which appears to be a disulfide-linked dimer of 13.5 k Da with two Arg-Gly-Asp sites), was found to trigger signaling events typically mediated by fibrinogen cross-linking of alpha(IIb)beta(3), a s demonstrated by tyrosine phosphorylation of the tyrosine kinase pp72 (syk) and a 140-kDa protein. Contortrostatin and another disintegrin, multisquamatin (a monomer of 5.7 kDa with a single Arg-Gly-Asp site), did not affect thrombin-induced platelet shape change, secretion, or i ntegrin-independent tyrosine phosphorylation; however, they inhibited aggregation and aggregation-dependent tyrosine phosphorylation of nume rous proteins, including the focal adhesion kinase pp125(FAK). Our res ults suggest that structurally distinct disintegrins have varying effe cts on tyrosine phosphorylation; while monomeric multisquamatin and di meric contortrostatin both inhibit aggregation-dependent tyrosine phos phorylation, contortrostatin also possesses a unique functional activi ty that allows it to activate an intracellular signaling pathway leadi ng to tyrosine phosphorylation. This activity may be involved in the f unction of this snake venom protein on hemostasis.